Combustion off-gases from the burning of fuels or the incineration of medical and other wastes contain a variety of chemical compounds known to be hazardous to human health. Among these combustion products are a number of specific polychlorinated organic compounds that are known to be carcinogenic at trace concentrations. The most toxic of these chlorinated hydrocarbons are 2,3,7,8 tetrachlorodibenzo-dioxin and the penta- chloro-, hexachloro-, heptachloro- and octachloro- homologs, hereinafter collectively designated as polychlorinated dibenzodioxins, PCDD, and the polychlorodibenzo-furan (PCDF) analogs, hereinafter collectively referred to as polychlorinated dibenzofurans, PCDF. In general, the lower number (tetra and penta) chlorinated congeners are the more carcinogenic to humans, and the octachloro congeners are conventionally held to be non-carcinogenic.
Efficient combustion at high temperatures, typically 1000.degree.-1200.degree. C., at adequate retention times, is known to destroy any PCDD/PCDF originally present in the fuel or wastes. However, detailed investigation of post-combustion chemical reactions has shown that the PCDD/PCDF form again on cooling down of the combustion gases. PCDD/PCDF are known to form from aromatic precursor compounds such as chlorobenzenes, chlorophenols, polychlorinated diphenyl ethers, polychlorinated biphenyls, benzo-furans, benzo-dioxins and the like, hereinafter collectively referred to as pre-dioxins and pre-furans.
In an article by Acharya, DeCicco and Novak, "Factors That Can Influence and Control the Emissions of Dioxins and Furans from Hazardous Waste Incinerators", published in the Journal of Air and Waste Management, pp. 1605-1615, Vol. 41, No. 12, 1991, the disclosure of which is hereby incorporated by reference, the authors state that the pre-dioxin and pre-furan compounds are believed to be formed at a temperature of about 500.degree. C. Dioxins then form downstream as the gases are further cooled, typically by an energy recovery boiler, in the range of approximately 250.degree. to 400.degree. C.
In the combustion of chlorine-containing fuels or wastes, hydrochloric acid gas, HCl, is invariably formed. In particular, the combustion of biomedical wastes (BMW) and municipal solid wastes (MSW) which contain relatively significant amounts of chlorine-containing plastics, such as polyvinyl chloride, produce appreciable amounts of HCl. Fundamental studies by Hoffman, Eiceman, Long, Collins, and Lu, "Mechanism of Chlorination of Aromatic Compounds Adsorbed on the Surface of Fly Ash from Municipal Incinerators", Environmental Science & Technology, pp. 1625-1641, Vol. 24, No. 11, 1990, have shown that the post-combustion formation of PCDD and PCDF takes place on the surfaces of fly ash by way of chlorination of adsorbed pre-dioxin and pre-furan compounds. Further, they showed that the active chlorinating agent is ferric chloride, FeCl.sub.3, formed by reaction of the HCl in the gas with the surface atoms of iron contained in the fly ash structure. Precursor adsorption equilibria is generally unfavorable at temperatures above 450.degree. C., which therefore limits the amount of PCDD/PCDF that can form by means of the fly ash adsorption/ferric chloride chlorination mechanism at the higher temperatures. The major portion of PCDD/PCDF forms as the temperature decreases to the level where adsorption on fly ash becomes greater than desorption equilibria, typically below 450.degree. C.
Lerner, in a paper entitled "Dioxin/Furan Removal: Negative Efficiency Behavior, Causes and Effects", presented at the 85th Annual Meeting of the Air & Waste Management Association, Kansas City, Miss., Jun. 21-26, 1992, the disclosure of which is hereby incorporated by reference, showed that PCDD/PCDF formation from the pre-dioxins and pre-furans proceeds to an extreme extent at the lower temperature range of 150.degree.-250.degree. C. if the fly ash holding the adsorbed pre-dioxins and pre-furans is retained for extended residence times in an HCl or FeCl.sub.3 -containing gas stream. Further, it was shown that in this temperature range, the lower molecular weight PCDD/PCDF congeners "distill" off into the gas phase from the adsorbed fly ash phase in the order of their relative fugacities. This results in an undesirable increase in the final exhaust gas content of the more toxic congeners, such as the tetra- and penta- chlorinated dioxins and furans; the higher chlorine number, and less toxic, congeners tend to remain on the fly ash.
U.S. Pat. No. 4,889,698 to Moller et al. use activated carbon for PCDD/PCDF removal. However, use of activated carbon to remove PCDD/PCDF after such compounds have been formed simply transfers the toxic PCDD/PCDF from the gas phase to the sorbent phase. The conventional use of activated carbon at low temperatures does not serve to reduce the total amount of PCDD/PCDF in the system (fly ash plus gas plus carbon) nor does it prevent its formation. Despite use of carbon at lower temperatures, the total amount of toxics formed remains fixed; only the distribution is changed. Additionally, the sorbent becomes contaminated with the highly toxic PCDD/PCDF. Disposal of the activated carbon is then a problem because the PCDD/PCDF-contaminated carbon is considered to be a hazardous material, and cannot be disposed of in sanitary landfills. The costs of the necessary disposal in a hazardous waste landfill are inordinately high. It would be desirable to avoid the costs of disposal of PCDD/PCDF contaminated sorbents or fly ash. It would also be desirable to avoid the formation of the PCDD and PCDF on the fly ash in the first place. However, methods and means of preventing adsorption of the precursor reactants on iron-containing fly ash have hitherto not been available.